The joining, infiltrating, or facing of metal work, or the manufacturing of many kinds of molded metal parts can be done efficiently (often with automation) using metalliferous powder-containing paste, provided, however, that upon heating and before fusion of the metal, the metal powder in the paste exhibits no significant hot slump (in other words, the powder stays substantially where the paste was deposited). This primary property has been difficult to obtain with conventional metal powder-containing paste. The instant vehicles and pastes can provide such property.
Restriction of flow of any resulting fused metal (that is a lack of general spreading about), except into intended joints or pores as in the joining of copper-based or iron-based metal parts, or in the infiltration of porous metal compacts such as steel compacts) is another property desired. The inventive products here can provide desired restriction of flow on fusion.
Brazing can be done with a paste of powdery powdered copper-based alloy or elemental copper-bearing, or with a cupreous so-called "silver solder". Facing applications, e.g., to coat, build-up or hard-face metal surfaces, can be done with pastes containing various metal or alloy powders, e.g., copper, copper-based, cobalt-based or nickel-based alloys, stainless steel, hard-facing alloys, etc. In hard facing, the metal powder usually is heated to a mushy state where liquid phase sintering of the metal occurs. Therefore, hard-facing powders can be considered here as essentially sinterable. Sometimes these pastes can contain particulate refractory materials such as alumina or silicon carbide for wear purposes. Infiltration of porous metal compacts, typically, steel compacts, call for fusion that will fill the pores of the compact with metal. Such pastes usually are copper-rich and contain a little iron, e.g., 0.01 to 6% by weight. Often a bit of refractory-providing material is used in them; the refractory material remains as a removable surface residue when the job is done. Molding applications, such as injection molding to make metal parts, can use a variety of metal powders in a paste vehicle. Most molded pastes here are formed of paste, then are heated to leave a sintered object. The vehicle weight proportion ordinarily is minor relative to that of the weight of fusible metal powder in most any of these foregoing pastes for efficiency and economy.
It has now been found that an improved vehicle for the instant pastes is provided by an essentially nonaqueous medium containing from 10-90 pbw hydrocarbon and a high surface tension nonaqueous organic liquid system. Such system provides an anti-slump property or slump control to the metal bearing paste such that when it is heated toward the melting point of the metal powder present, the shape of the deposit is substantially maintained. The hydrocarbon melts, of course, and wets the substrate around the deposit, but carries no significant amount of the metal particles with it. Apparently, the nonaqueous liquid system remains behind and provides a cohesive force for the powdered metal. Also, the hydrocarbon itself leaves no residue, is easily cleaned away from the joint or surrounding area. In the preferred compositions especially adapted for screen printing, no slump is observed. Surprisingly, even after melting, the fusible metal resists running from the site of application. The nonaqueous liquid system acts as an antislump agent. The compositions hereof are practically noncorrosive and easy to formulate and handle. They are readily adaptable to screen printing. Disadvantages attendant prior compositions, such as spattering due to evolution of water vapor, corrosivity, residue formation, etc., are not observed. The desirable properties of antislump and freedom from spattering in the metal--bearing pastes here is present and not in prior compositions known to me. Other additives are desirably present as will be pointed out below. The instant pastes have no water in them other than very minute amounts possible present as adventitious impurities that might be associated with good quality technical grade ingredients. No water is added deliberately as such, or as a hydrate, so it can be said that the instant pastes and vehicles are "nonaqueous" as a practical matter.